He entered the Department of Foreign Affairs, being appointed a Third secretary in 1974 and became a First Secretary in 1977. Later recruited by TaoiseachCharles Haughey, he has worked for the Fianna Fáil party ever since, serving under three Fianna Fáil leaders as Director of Research, Policy and Special Advisor on Northern Ireland where he was involved in discussions between the nationalist parties and the Irish Government and met regularly with intermediary Father Alec Reid.

In January 2009 he offered to quit his junior ministry post to save money and called on people to retain their Celtic Tiger style optimism and self-respect. He said: "We're not going to get anywhere by completely throwing overboard our self respect. We have achieved a tremendous amount in the past 20 years – they were the best 20 years in our history. There will be cycles – we rose very high and we are where we are now. We have to work our way out of this intelligently".

Mansergh has been a strong supporter of former TaoiseachBertie Ahern, whose financial affairs are under investigation by the Mahon Tribunal. He has been accused by some commentators of being insulting, condescending and petulant to opposition politicians.[6]

On an RTÉ radio[7] show Morning Ireland in February 2008, Mansergh reassured listeners that Ahern's difficulties were no more than a spot of inflight turbulence, with a safe landing in sight. When Fine Gael's tribunal expert, Senator Eugene Regan dissented, Mansergh became quite agitated, questioning why Regan wanted to question Ahern's finances declaring to Regan that: You should have respect for your betters![7]